Internal circuitry of computers can generate substantial amounts of heat. This is particularly so with rack-mount servers, which are often densely stacked within closed cabinets. As internal temperatures increase, the efficiencies and operating characteristics of internal components change. For example, transistors in processor and memory chip packages may exhibit changing operational characteristics which can lead to thermal runaway that permanently damages or destroys the components.
In order to maintain internal temperatures, most computer systems include one or more fans for cooling internal components. The fans may pump either ambient air or cooled air into the housing of the computer system. For example, a typical desktop computer may draw from ambient air to cool internal components. On the other hand, servers may draw from cooled air because the internal temperatures may be substantially higher than those reached in desktop computers.
Many computer facilities install large air cooling systems which direct cooled air to the servers via sub-floor spaces or specially designed Heating, Ventilation, and Air conditioning (HVAC) systems. In some facilities, cooled air may be piped directly into server rack systems or cabinets through vents in the sub-floor or through dedicated HVAC pipe systems.
In an effort to improve the efficiency of cooling systems, containment systems may be used to separate hot and cold air regions. For example, some systems may include chimney racks, rear door heat exchangers, specially designed containment racks, and aisle containment systems. One drawback of these containment systems is that pressure may build against the natural airflow of the server. During normal operation, the pressure build up is inconsequential to the server, because the normal operation of the server fans counteracts the typical pressure differentials.
By contrast, the pressure differential caused by containment systems can be an issue when a server is in an idle state. During idle state, the server's fan speeds and associated air flow rates are substantially reduced. Testing of a single rack unit (1 U) server suggests that air can start flowing backward at a hindering pressure of 0.08″ water (approx. 0.003 psi). Tests show that this value is easily reached in typical containment systems, particularly in a dense rack configuration. Furthermore, stressed systems can generate enough air movement to create high pressure differentials as seem by an idle server in the rack. Typically, the warm temperature blowback would have limited impact on the idle servers, but could have substantial impact on adjacent servers, which may cause fan speeds to increase to a point that would be noticeable by system users. Moreover, system efficiency would be reduced.